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A  Study  of  Some  New  Semi- 
permeable  Membranes. 


DISSERTATION 


SUBMITTED  TO  THE  BOARD  OF  UNIVERSITY  STUDIES  OF  THE  JOHNS 

HOPKINS  UNIVERSITY  IN  CONFORMITY  WITH  THE 

REQUIREMENTS  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY. 


BY 


J.  P.  COONY,  S.  J 


BALTIMORE, 

1903, 


A  Study  of  Some  New  Semi- 
permeable  Membranes. 


DISSERTATION 


SUBMITTED  TO  THE  BOARD  OF  UNIVERSITY  STUDIES  OF  THE  JOHNS 

HOPKINS  UNIVERSITY   IN  CONFORMITY  WITH  THE 

REQUIREMENTS  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY. 


BY 

J.  P.  COONY,  S.  J 


BALTIMORE, 
1903. 


CONTENTS. 


PAGB. 

Acknowledgment 4 

Introduction 5 

Historical  Sketch 5 

New  Membranes  by  Electrolysis 5 

Preparation  of  the  Cell  6 

Apparatus 7 

Electrical  Equipment 8 

The  Membrane :  Its  Position 10 

Formation  11 

Activity  and  Resistance 11 

Methods  of  Testing 12 

Ferric  Hydroxide 12 

Ferric  Phosphate 16 

Manganese  Ferrocyanide 17 

Deterioration 19 

Cobalt  Ferrocyanide 20 

Influence  of  Temperature  on  its  Activity 20 

Prussian  Blue 23 

General  Conclusions 25 

Leakage 27 

The  Cell  Wall 27 

Relations  between  Concentration  and  Rate  of  Flow 28 

Comparison  Curves 31 

Residual  Concentration .....  32 

Perfecting  the   Wall 34 

Addendum 36 

Biography 37 


186895 


ACKNOWLEDGMENT. 

To  Professor  Morse,  without  whose  initiative,  kindly 
assistance  and  experienced  direction  this  work  could  not 
have  been  carried  out,  the  author  wishes  to  make  grateful 
acknowledgment.  No  less  is  he  indebted  to  President 
Kemsen,  whose  lectures  have  been  both  guidance  and 
inspiration.  The  able  instruction  of  Professor  Jones,  Pro- 
fessor Shattuck  and  Professor  Clark,  likewise  calls  for 
appreciative  recognition. 


A  STUDY   OF    SOME   NEW   SEMIPEKMEABLE 
MEMBRANES. 

It  is  quite  unnecessary,  in  giving  an  account  of  work 
done  on  the  subject  which  forms  the  title  of  this  paper,  to 
review  even  briefly,  the  work  and  measurements  of  Pfeffer,1 
the  brilliant  theoretical  work  of  van  't  Hoff 2  founded  on 
them,  or  the  extremely  valuable  results  achieved  by  De 
Vries,3  Tammann,4  and  Hamburger,5  men  who  were  stimu- 
lated to  undertake  their  great  work  by  van  'tHoff's  con- 
clusions. The  great  importance  of  the  subject  has  made 
the  scientific  world  familiar  with  these  results.  And  if 
subsequent  progress  has  failed  to  correspond  to  these  bril- 
liant beginnings,  the  explanation  is  to  be  sought  for  in  the 
almost  insurmountable  difficulties  everywhere  to  be  met 
with. 

The  failure  of  so  many  attempts  to  repeat  Pfeffer's  work 
demonstrated  the  fact  that  only  by  great  good  fortune 
could  an  experimenter  hope  to  secure  a  good  membrane 
and  a  reliable  cell  by  the  methods  employed  by  this  investi- 
gator. Indeed,  no  work  comparable  to  his  had  been  done 
until  a  new  method,  devised  by  Morse6  and  Horn,  and 
further  perfected  by  Morse  7  and  Frazer,  gave  well  founded 
hopes  of  easily  and  surely  forming  quite  perfect  mem- 
branes. The  results  attained  by  these  last  workers  are 
most  interesting  indeed,  as  they  show  the  great  rapidity 
with  which  a  satisfactory  membrane  develops  a  pressure 
sufficient  to  burst  very  highly  resisting  materials.  Near 

Osmotische  Untersuchung  (1877) . 
Zeitsch.  phys.  Chem.  I.  481  (1887). 
Ibid.  2,  415  (1888). 
Wied.  Ann.  34,  299  (1888). 
Zeitsch.  phys.  Chem.  6,  319  (1890V 
Amer.  Chem.  Journ.26,  80  (1901). 
Ibid.  28,  1  (1902). 


—  6  — 

the  end  of  the  paper  last  mentioned  we  read  the  brief  his- 
tory of  a  cell  in  which  the  measurement  of  the  osmotic 
pressure  of  a  normal  solution  of  sugar  was  attempted. 
"  Resistance  of  the  membrane  not  exactly  known,  but 
over  200,000  ohms.  The  solution  of  sugar  fresh  and  free 
from  invert  sugar.  Cell  set  up  at  5  P.  M.  Pressure  at 
5.10  P.  M.,  7.89  atmospheres;  at  6.45  P.  M.,  31.41 
atmospheres  (corrected).  Temperature  24°. 9.  Shortly 
after  the  second  reading  the  glass  tube  was  shattered." 

The  very  satisfactory  results  recorded  in  this  last  paper 
gave  a  new  impetus  to  work  in  this  field  and  the  prospect  of 
soon  being  able  to  measure  directly  and  with  accuracy  the 
osmotic  pressure  of  normal  solutions,  and  probably  of  vet 
higher  concentrations,  made  clear  the  immediate  desirability 
of  a  number  of  semipermeable  membranes  of  a  sufficiently 
wide  range  of  chemical  characteristics  to  permit  of  the 
selection,  in  each  individual  case,  of  some  one  membrane 
which  should  be  chemically  inert  with  regard  to  the  par- 
ticular substance  whose  osmotic  pressure  measurements 
were  desired.  For  the  discovery  of  such  membranes  the 
electrolytic  method  seemed  to  promise  speedy  and  sure  re- 
sults, and  it  was  accordingly  made  use  of  in  the  following 
investigation. 

In  testing  the  various  semipermeable  membranes  investi- 
gated in  this  laboratory,  two  cups  have  been  employed.  The 
smaller  kind  has  a  capacity  of  about  15  cc.  and  seems  in 
general  to  possess  a  finer  texture  and  to  have  been  burned  to 
a  greater  degree  of  hardness  than  the  larger  variety,  the 
capacity  of  which  varies  from  170  to  200  cc.  This  larger 
cell,  because  of  its  shape,  is  commonly  called  the  bottle  cell. 

THE  PRELIMINARY  PREPARATION  OF  THE  CELL. 

This  preliminary  treatment  consists  first  in  melting 
shellac  within  the  neck  of  the  cell  until  a  firm  coating  is 
secured,  covering  all  that  portion  of  the  cell  wall  which 


could  be  affected  by  the  introduction  and  removal  of  the 
rubber  stoppers  employed.  The  purpose  of  this  film  of 
shellac  is  to  make  sure  that  no  portion  of  the  semiper- 
meable  membrane  formed  will  be  exposed  to  the  danger  of 
rupture  by  friction  or  by  the  strains  set  up  in  the  neck  of 
the  cell  by  the  tightly  fitting  stoppers.  By  this  means  is 
precluded  all  possibility  of  a  leakage  of  the  solutions 
through  portions  of  the  porous  wall  where  either  no  mem- 
brane has  been  formed,  or,  if  formed,  has  been  ruptured  or 
locally  removed  in  the  process  of  closing  the  cell. 

The  next  step  is  to  free  the  cell  wall  of  all  soluble  mate- 
rials, especially  of  the  air  contained  within  the  pores.  As 
the  simple  apparatus  here  employed  is  used  also  in  the 
formation  of  the  membrane,  it  will  be  well  to  describe  it  in 
detail.  A  rubber  stopper  through  which  passes  the  direct 
arm  of  a  glass  T  tube  of  say  10  mm.  internal  diameter  is 
fitted  into  the  neck  of  the  cell.  This  admits  a  second  glass 
tube  of  three  or  four  mm.  diameter.  This  second  tube, 
the  purpose  of  which  is  the  transmission  of  liquids  into 
the  cell,  reaches  quite  to  the  bottom  of  the  cell,  and  is  the 
outlet  from  a  reservoir  of  suitable  capacity.  The  flow 
from  this  into  the  cell  can  be  regulated  at  will  by  a  pinch 
cock  on  the  rubber  connection.  Surrounding  the  lower  end 
of  this  second  tube  is  the  inner  platinum  electrode,  the 
platinum  connection  of  which  passes  also  through  the  T 
tube.  The  horizontal  arm  of  the  latter  serves  as  an  escape 
for  the  liquids  introduced  into  the  cell  from  the  reservoir. 
By  this  means  the  liquid  within  the  cell  can  be  renewed  as 
desired  without  any  interruption  to  the  process.  In  most 
cases  frequent  renewal  is  advisable.  The  cell  is  then  placed 
in  a  beaker,  of  a  depth  sufficient  to  permit  of  complete 
immersion,  and  is  surrounded  by  the  outer  electrode.  In 
the  process  of  washing  cells,  and  frequently  also  in  building 
up  the  membrane,  this  electrode  is  of  platinum,  though  in 
the  latter  process,  other  workers  in  this  laboratory  have 


—  8  — 

found  electrodes  of  Cu,  Ni,  Sn,  Zn,  etc.,  to  be  more  satis- 
factory when  the  membrane  to  be  deposited  was  a  salt  of 
one  of  these  metals.  When  the  membrane  is  some  insolu- 
ble ferric  salt,  both  electrodes  must  be  of  platinum,  as  a 
test  made  with  an  iron  anode  showed  that  by  the  electrolytic 
action  an  appreciable  portion  of  ferrous  compound  was 
formed  together  with  the  ferric  salt. 

The  cell  walls  should  first  be  thoroughly  wetted  from 
either  the  outside  or  the  inside.  This  is  done  in  order  to 
avoid  the  probable  entrapping  of  air  between  the  advancing 
walls  of  liquid  should  the  latter  be  applied  to  both  sides 
of  the  cell  wall  at  about  the  same  time.  When  thus  wet- 
ted, both  the  cell  and  the  beaker  in  which  it  stands  are  filled 
with  a  0.05%  solution  of  potassium  sulphate  in  recently 
boiled  distilled  water.  With  such  apparatus  the  process 
of  washing  is  easilv  carried  out.  The  inner  electrode  is 

o  J 

made  the  cathode  and  the  current  intensity  so  regulated 
that  the  endosmose  amounts  to  8  or  10  cc.  per  minute. 
The  outer  solution  needs  replenishing  at  times  to  supply 
loss  and  to  keep  the  cell  covered,  and  both  outer  and  inner 
solutions  require  occasional  renewal,  as  they  shortly 
become  weak  in  cations  and  anions  respectively.  Two 
hours  of  this  treatment  is  followed  by  a  similar  treatment 
with  boiled  distilled  water.  The  current  is  passed  until  a 
very  low  conductivity  shows  the  nearly  complete  absence 
of  potassium  sulphate.  This  leaves  the  cell  walls  quite 
free  of  foreign  materials,  and  the  membrane  may  now  be 
introduced,  or  the  cell  may  be  kept  in  distilled  water  until 
the  operator  is  ready  to  proceed  to  this  step. 

THE    FORMATION    OF    THE    SEMIPERMEABLE    MEMBRANE. 

The  Electrical  Equipment.  —  The  current  employed  is 
usually  taken  from  the  storage  batteries  whose  combina- 
tions permit  the  use  of  any  potential  between  2  and  110 
volts.  Connections  are  easily  established,  also,  with  two 


dynamo  circuits  of  110  and  220  volts  respectively;  while 
three  rotary  transformers  furnish  currents  at  any  desired 
potential  below  300  volts.  In  the  large  and  coarser  cells 
low  voltages  are  always  found  necessary  in  the  earlier  stages 
of  the  process;  frequently  12,  and  at  times  even  6 
volts  marking  the  upper  limit  of  the  current  which  may 
safely  be  employed.  In  such  cases,  the  use  of  a  stronger 
current  seems  to  prevent  any  rise  in  the  electrical  resist- 
ance of  the  membrane.  It  is  thought  that  under  such 
conditions  the  membrane  is  unable  to  bridge  over  many 
of  the  pores  in  the  cell  wall,  which,  by  a  less  vigorous 
treatment  might  be  effectually  closed.  Thus  a  large  and  un- 
varying cross-section  of  the  liquid  is  left  for  the  unimpeded 
passage  of  the  ions.  The  safer,  and,  indeed,  more  expedi- 
tious method  seems  to  be  to  confine  the  current  to  limits  of 
one,  or  at  most  two-tenths  of  an  ampere,  raising  the  volt- 
age somewhat  as  the  resistance  of  the  membrane  rises; 
though  an  exception  seems  to  be  necessary  in  the  case  of 
the  Prussian  Blue  membrane.  With  this  membrane  no 
good  results  were  obtained  until  either  a  small  current  for 
a  long  period,  or  a  large  current  for  a  shorter  time  had 
almost  choked  the  cell  walls  with  the  amount  of  the  pre- 
cipitate formed. 

The  Solutions.  — The  sulphates  or  nitrates  of  the  metals 
were  preferably  employed.  It  is  evident  that  when  plati- 
num anodes  are  used  the  chlorides  are  necessarily  barred. 

Concentrations.  — Though  varying  conditions  have  made 
changes  in  concentration  advisable,  the  practice  of  Morse 
and  Horn  and  of  Morse  and  Frazer  of  generally  using 
N/10  solutions  has  been  found  to  give  the  best  results. 
Theoretically,  this  question  of  concentration  should  be 
determined  by  a  consideration  of  the  relative  dissociations 
of  the  electrolytes  at  the  concentrations  employed,  as  also 
of  the  relative  ionic  velocities  of  the  two  constituents  of  the 
membrane.  But  the  practical  solution  is  much  more  easily 


carried  out,  and  consists  in  simply  observing  the  position 
of  the  membrane  —  whether  within,  or  upon  either  edge  of 
the  porous  wall  —  and  in  changing  the  relative  concentra- 
tions accordingly. 

While  no  systematic  study  has  been  made  of  the  advan- 
tages or  disadvantages  of  the  various  positions  in  which  the 
membrane  may  be  deposited,  it  is  obvious  that  a  membrane 
formed  on  the  outer  edge  of  the  wall  can  have  no  support 
whatever  against  pressures  from  within.  The  fact  is,  that 
cells  in  which  the  membrane  has  been  deposited  upon  the 
inner  edge  of  the  wall  have  been  found  to  be  uniformly 
much  more  active  than  those  in  which  the  membrane  was 
formed  within  the  wall.  This  was  to  be  expected,  as  diffu- 
sion takes  place  comparatively  slowly  within  a  porous  body 
of  close  texture  and  considerable  density ;  and  instead  of 
having  the  semipermeable  membrane  as  a  fine  partition 
sharply  separating  pure  water  from  a  solution  of  say  normal 
concentration,  the  incoming  water  would  soon  so  dilute  the 
solution  in  the  pores  of  the  wall  that  the  concentration  at 
the  membrame  would  probably  fall  far  below  one-tenth 
normal,  and  the  activity,  as  indicated  by  the  rate  of  flow  of 
the  water  through  the  membrane,  would  be  correspondingly 
low.  If,  on  the  contrary,  the  semipermeable  membrane  is 
on  the  inner  edge  of  the  wall,  the  practically  undiminished 
concentration  of  the  solution  within,  and  the  pure  water 
circulating  through  the  porous  wall,  are  separated  only  by 
the  thin  membrane,  and  the  maximum  flow  will  be 
observed.  In  the  matter  of  measuring  osmotic  pressures 
also  —  and  for  the  same  reason — a  maximum  pressure 
could  hardly  be  expected,  even  after  a  very  long  time,  if 
the  membrane  be  within  the  wall. 

In  consequence  of  these  considerations,  efforts  have  been 
made  to  so  correlate  the  concentrations  of  the  two  electro- 
lytes that  the  semipermeable  membrane  should  form  upon 
the  inner  edge  of  the  wall;  and  as  experiment  has  shown 


—  11  — 

that  by  using  high  relative  concentrations  within,  the  pre- 
cipitate can  be  formed  even  in  the  outer  electrolyte,  and 
vice  versa,  the  matter  of  position  is  under  complete  con- 
trol. In  practice,  a  beginning  is  made  with  concentrations 
probably  correct,  regard  having  been  had  for  ionic  veloci- 
ties and  dissociations  of  the  electrolytes,  and,  if  the  mem- 
brane forms  where  it  is  desired,  as  is  commonly  the  case, 
no  change  is  made.  Otherwise,  one  or  the  other  solution 
is  diluted  accordingly. 

TO    FORM    THE    SEMIPEEMEABLE    MEMBRANE. 

When  the  cell  has  been  prepared  and  the  apparatus  set 
up  as  before  described,  connection  is  made  with  the 
proper  electrical  terminals,  and  the  electrolytes  are  in- 
troduced nearly  simultaneously.  The  electrical  resistance 
falls  during  the  short  period  required  for  the  electrolytes 
to  displace  the  water  from  the  pores  of  the  wall  and  rises 
as  soon  as  the  formation  of  the  membrane  begins.  Under 

o 

proper  conditions,  the  resistance  continues  to  increase 
until  a  maximum  has  been  reached  which  corresponds  to 
the  character  of  the  porous  wall  and  the  capabilities  of  the 
particular  membrane.  These  observations  coincide  with 
conclusions  reached  by  Morse  and  Horn,  who,  working 
with  cups  of  considerable  porosity,  found  that  the  resist- 
ance of  the  membrane  could  not  be  raised  above  a  certain 
point.  This  maximum  seems  to  be  determined  by  the 
number  and  size  of  those  pores  which  the  membrane  is 
unable  to  bridge. 

If,  after  a  delay  of  some  days,  the  membrane-forming 
process  is  repeated,  a  considerably  higher  maximum  re- 
sistance is  usually  obtained.  This  second  maximum  is 
seldom  exceeded  in  later  repetitions,  even  though  these  be 
quite  numerous.  If,  however,  the  concentrations  be  so 
changed  as  to  form  the  membrane  in  some  new  position, 
the  resistance  becomes  quite  irregular.  Such  irregularity 


—  12  — 

was  very  marked  in  the  earlier  work.  Instances  also 
occurred  of  the  resistance  falling  considerably  below  the 
maximum  by  the  mere  continuance  of  the  process.  This 
phenomenon  is  thus  far  without  a  satisfactory  explana- 
tion. 

TESTING    THE    MEMBRANE. 

After  the  membrane  has  been  formed  the  cell  is  washed 
thoroughly  with  distilled  water,  filled  with  the  desired 
solution  —  usually  a  sugar  solution  exactly  normal  —  and 
closed  with  a  tightly  fitting  rubber  stopper  containing  either 
a  manometer  or  a  simple  delivery  tube.  It  is  then  piuced  in 
a  vessel  containing  sufficient  distilled  water  to  rise  above 
the  highest  level  reached  by  the  membrane.  The  mem- 
brane may  be  tested  for  activity,  that  is,  for  the  rate  of  flow 
of  the  pure  solvent  through  the  membrane  into  the  solu- 
tion within ;  or,  it  may  be  tested  for  the  pressure  developed 
with  a  given  concentration.  In  the  former  case  the  delivery 
tube  is  so  arranged  that  its  lowest  point  is  about  10  mm. 
above  the  level  of  the  outer  liquid.  The  cell  is,  in  conse- 
quence, always  operating  against  this  slight  pressure.  If 
it  is  desirable  to  test  the  membrane  for  pressure —  and  this 
was  done  only  in  the  early  part  of  the  work  —  the  rubber 
stopper  contains  a  closed  manometer  in  which  the  com- 
pression of  a  volume  of  carefully  purified  air  indicates  the 
pressure  developed. 

THE  VARIOUS    MEMBRANES. 

The  Ferric  Hydroxide  Membrane. — The  properties  of 
this  well-known  precipitate,  ferric  hydroxide,  seem  such  as 
to  make  it  suitable  in  the  highest  degree  for  a  semiperme- 
able  membrane.  Its  insolubility,  its  firm  gelatinous  con- 
sistency, are  almost  too  well  known  to  all  those  who  have 
attempted  the  gravimetric  determination  of  iron.  The 
difficulty  experienced  in  washing  the  precipitated  hydroxide 


—  13  — 

at  once  suggests  the  idea  of  impermeability  with  regard  to 
dissolved  materials.  Its  chemical  inertness  with  regard  to 
most  basic  substances  strongly  recommend  it  for  the  meas- 
urement of  the  osmotic  pressure  of  this  class  of  compounds. 

As  the  only  ferric  salt  immediately  obtainable  was  the 
chloride,  which  is  manifestly  unsuitable  for  use  with  plati- 
num electrodes,  the  first  attempt  was  made  with  ferrous 
sulphate,  giving  a  membrane  of  ferrous  hydroxide.  The 
expansion  of  this  upon  oxidation  to  the  ferric  hydroxide, 
would,  it  was  thought,  produce  an  extremely  close  semi- 
permeable  membrane.  A  bottle  cell  of  185  cc.  capacity 
was  filled  with  a  N/15  solution  of  ferrous  sulphate,  to 
which  was  added  a  considerable  amount  of  carefully 
washed  ferric  hydroxide  precipitate,  introduced  for  the 
purpose  of  taking  up  the  sulphuric  acid  set  free  by  the 
electrolysis.  Outside,  a  N/30  solution  of  sodium  hydrox- 
ide was  employed.  This  lower  concentration  was  thought 
sufficient  because  of  the  much  higher  velocity  of  the 
hydroxyl  ions.  The  membrane  began  forming  almost  im- 
mediately, and  with  a  potential  of  S3  volts  the  resistance 
of  the  cell  increased  quite  regularly  at  the  rate  of  nearly 
20  ohms  per  minute,  reaching  825  ohms  in  forty-five  min- 
utes. The  rise  in  resistance  was  less  rapid  after  this,  and 
over  an  hour  was  required  to  reach  the  maximum  of  1000 
ohms.  The  resistance  an  hour  later  marked  800  ohms. 

The  cell  was  next  thoroughly  washed  with  distilled 
water,  then  immersed  in  the  same  liquid,  while  through 
the  interior  air  bubbles  were  continuously  forced  during 
eighteen  hours.  How  complete  was  the  oxidation  was 
never  known.  The  membrane,  however,  never  exhibited 
any  osmotic  activity,  and  attention  was  next  turned  to- 
wards the  preparation  of  pure  ferric  salts;  the  sulphate, 
the  nitrate  and  the  acetate.  When  these  were  obtained 
cell  II  of  the  same  kind  was  filled  with  a  N/2  ferric  sul- 
phate solution,  while  the  outer  liquid  was  a  N/12  sodium 


—  14  — 

hydroxide  solution.  Here  the  resistance  when  the  mem- 
brane first  began  to  form  was  12  ohms.  In  45  minutes 
it  rose  to  385  ohms.  The  cell  was  then  allowed  to  stand 
over  night  in  distilled  water,  the  formation  of  the  mem- 
brane being  resumed  on  the  following  morning.  The 
resistance  now  rose  rapidly,  going  beyond  1000  ohms 
within  thirty -five  minutes.  At  this  point  the  cell 
was  washed,  and  then  set  up  with  a  normal  sugar 
solution,  but  without  any  manifestation  of  activity.  The 
formation  of  a  successful  membrane  in  this  cell  was  next 
attempted  from  ferric  nitrate.  The  resistance  of  the  mem- 
brane reached  a  maximum  of  2680  ohms  one  hour  after  start- 
ing, and  in  the  next  fifteen  minutes  dropped  to  2300  ohms. 
Here  again,  the  cell  was  washed,  and  again  set  up  with  the 
normal  sugar  solution,  but  without  a  more  favorable  result 
than  before.  In  the  third  attempt,  the  concentration  of 
the  sodium  hydroxide  solution  was  made  equal  to  that  of 
the  ferric  nitrate  within.  This  gave  a  low  resistance 
varying  not  far  from  100  ohms,  and  the  fast  moving 
hydroxyl  ions,  passing  entirely  through  the  wall,  formed  an 
abundant  precipitate  of  loose  ferric  hydroxide  inside  the  cell. 
A  last  attempt  was  made  employing  ferric  acetate.  The 
highest  resistance  obtained  was  175  ohms.  The  cell 
showed  no  osmotic  activity  with  either  a  normal  sugar 
solution  or  a  twice  normal  solution  of  sodium  chloride. 
One  week  later  the  outer  liquid  was  intensely  salt;  from 
which  we  infer  that  the  ferric  hydroxide  membrane,  in  such 
a  wall  is  completely  permeable  with  regard  to  this 
substance. 

Cell  XVIII.  —  This  cell,  of  the  same  grade,  was  tried 
six  weeks  later  when  considerable  experience  had  been 
gained  with  other  membranes  in  the  same  type  of  cell. 
Three  days  were  spent  in  futile  efforts  to  form  an  active 
membrane  of  ferric  hydroxide.  The  cell  when  set  up  with 
a  normal  sugar  solution  showed  no  activity,  and  twenty- 


—  15  — 

four  hours  later  the  marked  sweetness  of  the  outer  solu- 
tion proved  either  that  this  kind  of  cell  does  not  properly 
support  a  ferric  hydroxide  membrane,  or  that  this  mem- 
brane is  permeable  as  well  towards  sugar  as  towards  sodium 
chloride. 

With  the  small  cells  of  finer  texture  the  ultimate  result 
was  widely  different.  In  two  hours  with  a  N/10  solution 
of  ferric  acetate  within  and  a  N/20  solution  of  sodium 
hydroxide  without,  the  resistance  rose  to  9000  ohms  — 
the  maximum  resistance  developed  in  this  cell.  This  was, 
comparatively  speaking,  a  lower  figure  than  those  obtained 
in  the  larger  cells ;  regard  being  had  for  the  correspond- 
ingly large  cross-section  of  conductive  membrane.  Yet, 
when  set  up  with  a  N/2  sugar  solution,  the  liquid  rose 
rapidly  to  the  top  of  the  open  manometer  tube.  This  had 
a  length  of  1.2  m.  and  a  bore  of  about  1  mm.  The  rate 
of  rise  of  the  liquid  was  about  10  mm.  per  minute.  This 
cell  continued  to  flow  freely  during  the  next  three  days. 
It  was  then  taken  down  and  fitted  for  measuring  higher 
pressures.  The  membrane  was  reinforced  by  a  further 
electrolytic  deposition  of  ferric  hydroxide,  and  the  cell, 
after  having  been  filled  with  a  normal  sugar  solution,  was 
fitted  with  a  closed  manometer  and  placed  in  the  distilled 
water.  The  highest  pressure  recorded  was  0.5  atmos- 
pheres. This  seemed  to  indicatete  that  the  insufficiently 
supported  membrane  gave  way  locally,  allowing  the  leak- 
age at  this  pressure  to  equal  the  inflow  of  water  The 
known  activity  of  the  membrane  and  the  amount  of  sugar 
passing  into  the  outer  water  could  not  otherwise  be  ac- 
counted for. 

A  second  cell  of  this  type,  which  had  failed  to  produce 
an  active  membrane  by  a  similar  treatment  with  ferric  phos- 
phate, was  tried  with  the  ferric  hydroxide  membrane. 
This  cell  when  a  resistance  of  3000  ohms  had  been 
reached,  was  set  up  with  an  open  manometer  tube  2.4  m. 


—  16  — 

in  height.  The  level  of  liquid  in  this  tube  rose  at  the 
very  rapid  rate  of  92  rnm.  per  minute  when  at  the  height 
of  600  mm.,  and  the  sugar  solution  flowed  freely  from 
the  top  of  the  tube  during  the  two  days  it  was  allowed 
thus  to  remain.  Four  other  cells  of  the  same  type 
were  tried  with  this  membrane,  the  results  being  uniformly 
satisfactory  as  regards  activity,  but  never  showing  a  pres- 
sure much  in  excess  of  1  atmosphere.  Of  these  cells  the 
flow  of  one  was  measured.  The  capacity  of  the  cell  was 
16  cc.  and  it  was  delivering  at  a  pressure  of  about  35  mm. 
water.  The  figures  record  the  total  delivery  after  twenty- 
four  hour  periods. 

First    24  hours 8.0  cc. 

Second       "        13.2  " 

Third          "        16.2  " 

Fourth       "        18.4  " 

These  cells  were  afterwards  broken  and  their  membranes 
carefully  examined.  They  appeared  firmer  and  more  uni- 
form than  the  other  membranes  tested  in  this  investigation. 
There  is  every  probability  that  in  a  porous  wall  of  a  suffi- 
ciently fine  and  uniform  texture  this  membrane  will  prove 
to  be  of  great  value.  The  cells  thus  far  tried  with  it  are 
known  to  give  unreliable  support  under  pressure :  indeed, 
the  cells  of  the  coarser  type  failed,  even  when  no  pres- 
sure was  developed ;  and  hence  we  may  reasonably  ascribe 
to  them  and  not  to  the  membrane  the  deficiencies  mani- 
fested. 

The  Ferric  Phosphate  Membrane. — A  fair  trial  of  this 
membrane  cannot,  it  is  probable,  be  made  until  a  cell  wall  of 
the  desired  texture  is  forthcoming.  Two  cells  of  the  finer 
grade  were  given  careful  trials,  yet  they  failed  to  indicate 
any  osmotic  action  whatever.  These  trials  were  not  suffici- 
ently exhaustive  to  prove  the  complete  incapacity  of  these 
cells  for  supporting  the  membrane,  yet  it  is  clear  that  the 


—  17  — 

membrane  could  not  be  satisfactory  in  them.  The  appear- 
ance of  the  ferric  phosphate  precipitate  is  most  promising, 
and  its  chemical  character  such  as  to  make  it  extremely 
valuable  if  an  active  membrane  can  be  formed  from  it. 

The  Manganese  Ferrocyanide  Membrane. — This  precip- 
itate is  of  fine  grain  with  very  little  tendency  toward  ag- 
glutination, and  it  does  not  offer  the  appearance  of  a  prom- 
ising semipermeable  membrane.  The  color,  white  at  first, 
changes  on  standing  to  a  delicate,  pale  green.  This  change 
occurs  as  well  in  the  cell  when  filled  with  a  sugar  solution  as  in 
any  vessel  in  which  the  precipitate  maybe  formed.  In  de- 
positing this  membrane  it  has  always  been  found  necessary 
to  employ  considerably  lower  voltages  than  with  other 
membranes  under  similar  conditions.  The  highest  poten- 
tial which  could  be  safely  used  in  the  first  application  of 
the  membrane-forming  process  to  a  cell  of  the  coarser 
variety  was  12  volts,  nor  was  any  higher  voltage  deemed  safe 
when,  after  a  four  days'  test  with  a  normal  sugar  solution, 
a  reinforcement  of  the  membrane  was  found  necessary.  In 
the  third  treatment  of  this  cell,  four  weeks  later,  any  at- 
tempt to  go  beyond  33  volts  at  once  lowered  the  resistance. 
This  fall  of  resistance  continued  until  a  return  was  made 
to  the  lower  voltage. 

This  membrane  was  tried  in  cells  of  both  types,  audits 
behavior  was,  in  general,  unsatisfactory.  Its  activity  as 
manifested  by  the  rate  of  flow  under  a  pressure  of  10  mm. 
of  water  was  about  one -half  that  of  the  ferric  hydroxide 
membrane,  about  one-third  that  of  the  cobalt  ferrocyanide 
membrane,  and  two-thirds  that  of  the  Prussian  Blue  mem- 
brane under  similar  conditions.  This  comparison  in  the 
case  of  the  ferric  hydroxide  membrane  could  be  made 
only  in  the  smaller  cells ;  in  the  other  cases  the  larger  cells 
were  employed. 

The  first  cell  tried  was  one  of  the  larger  variety.  The 
solutions  were  N/10  manganese  sulphate  and  potassium 

2 


—  18  — 

ferrocyanide,  and  the  current  from  the  battery  with  a 
potential  of  12  volts  was  passed  continuously  during  two 
hours  and  forty -five  minutes ;  a  maximum  resistance  of  345 
ohms  being  reached  at  the  end  of  two  hours  of  the  electro- 
lytic action.  After  washing,  the  cell  was  set  up  with  a 
normal  sugar  solution  to  deliver  at  a  pressure  of  10  mm. ; 
the  temperature  being  about  15°.  The  flow  at  first  was 
at  the  rate  of  over  5  cc.  per  hour,  but  this  activity  soon 
diminished,  reaching,  in  three  and  one-half  days,  the  small 
figure  of  9.1  cc.  per  day,  with  a  total  delivery  of  65  cc. 
Here  the  membrane-forming  process  was  repeated,  a  resist- 
ance of  1167  ohms  being  reached  in  one  hour.  At  first 
the  rate  of  delivery  was  slower  than  before,  but  the  staying 
qualities  of  the  firmer  membrane  were  plainly  manifest. 
Two  weeks  later  with  a  total  delivery  of  177.5  cc.  the 
rate  was  still  9  cc.  per  day;  though  frofn  this  time  on  the 
diminution  was  marked,  only  3.7  cc.  being  delivered  in  the 
same  period  ten  days  later;  the  total  delivery  in  twenty-four 
days  being  233.4  cc. 

The  third  attempt  to  build  up  a  firm  membrane  in  this 
cell  showed  no  improvement  over  the  second.  The  resist- 
ance was  nearly  the  same,  the  activity  of  the  membrane 
somewhat  less  than  before,  and  the  total  amount  delivered 
in  thirty-seven  days  only  224.5  cc.  A  fourth  and  fifth 
time  similar  efforts  were  made  to  reinforce  this  membrane. 
The  activity  as  indicated  by  the  rate  of  its  measured  de- 
livery seemed  somewhat  diminished  each  time,  while  the 
resistance  remained  practically  the  same.  A  peculiarity  of 
this  cell  was  that  after  it  had  been  set  up  for  two  or  three 
days,  the  sugar  solution  showed  a  slightly  yellowish  tint 
and  contained  appreciable,  and  constantly  increasing 
amounts,  of  very  fine  white  suspended  matter,  not  distin- 
guishable from  the  magnanese  ferrocyanide.  In  sixteen  days 
this  turbid  condition  was  quite  marked  both  in  the  water 
surrounding  the  cell  and  in  the  solution  within.  The  latter 


—  19  — 

solution  had  become  quite  opaque.  This  characteristic  was 
persistent  throughout  the  whole  history  of  the  cell,  not 
only  in  its  numerous  trials  with  sugar  solutions  but  also  on 
being  tested  with  alcohol.  Here  the  delivery  was  -of  a  very 
marked  yellow  color. 

In  other  cells  the  yellowish  tint  was  somewhat  in  evi- 
dence, but  not  the  turbid  character  of  the  liquids  within 
and  without.  Cell  XXII.  of  the  same  kind  and  with  the 
same  membrane,  duplicated  in  every  respect,  except  the 
last,  the  results  just  recorded.  Two  tests  with  cells  of  the 
finer  grade  showed  nothing  more  than  that  the  membrane 
was  better  supported  in  these  cells  and  that  it  continued 
longer  in  them  without  deterioration.  In  one  of  these  cells 
the  curve  representing  the  rate  of  delivery  as  plotted  against 
concentration  showed  no  appreciable  deterioration  of  the 
membrane  during  thirty-two  days ;  and  when,  thirty  days 
later,  the  cell  was  broken  for  the  inspection  of  the  membrane 
the  latter  was  quite  equal  in  appearance  to  the  sound  and  firm 
ferric  hydroxide  membrane.  With  this  single  exception, 
all  the  ferrocyanide  membranes  tested  showed  some  dete- 
rioration. This  was  best  observed  when  the  cell,  after  hav- 
ing been  subjected  to  the  desired  number  of  tests,  was 
broken,  and  the  membrane  compared  in  appearance  with  a 
newly  formed  membrane  of  the  same  type.  It  was  also 
perceived  by  a  falling  off  in  the  delivery  of  these  cells  in 
the  later  periods  of  their  history.  This  deterioration  con- 
sisted mainly  in  a  partial  transformation  of  the  ferrocyanide 
into  the  corresponding  oxide,  though  complete  local 
removal  of  the  membrane  also  took  place.  In  the  manganese 
ferrocyanide  membrane  the  deterioration  was  chiefly  of 
the  latter  kind  and  was  very  marked,  being  greater 
than  in  any  other  membrane.  In  the  cobalt  ferrocyanide 
membrane  both  forms  of  deterioration  took  place,  but  in  a 
lesser  degree ;  while  in  the  Prussian  Blue  membrane  only 
local  removal  seems  to  have  occurred,  and  this  to  no  great 
extent  in  the  very  thick  membranes. 


—  20  — 

The  Cobalt  Ferrocyanide  Membrane.  — The  next  mem- 
brane studied  was  that  precipitated  from  cobalt  sulphate 
and  potassium  ferrocyanide.  This  precipitate  is  flocculent, 
and  quite  cohesive.  When  freshly  precipitated,  it  is  green- 
ish in  tint,  changing  to  a  bluish-green,  and,  in  very  old 
membranes,  to  a  purple  color.  There  has  never  been  any 
difficulty  in  forming  a  membrane  from  this  precipitate. 
With  N/10  solutions  of  these  salts  and  a  battery  current 
of  12.4  volts,  the  resistance  of  the  membrane  in  one 
of  the  larger  cells  rose  steadily,  reaching  910  ohms  at  the 
end  of  two  hours  and  thirty  minutes.  When  set  up  with 
a  normal  sugar  solution — the  temperature  being  that  of 
the  laboratory  and  averaging  between  15°  and  16° —  the 
membrane  showed  an  activity  notably  greater  than  any 
hitherto  observed.  Without  having  been  reinforced,  this 
membrane  continued  to  manifest  the  same  high  degree  of 
activity  throughout  the  entire  period  of  its  first 
test.  At  the  end  of  fifty-five  days  the  delivery 
amounted  to  4.4  cc.  per  day,  the  total  delivery  had  been 
574  cc.,  and  the  concentration  of  the  sugar  solution 
within  had  dropped  to  N/61.  With  no  further  treatment 
than  washing  with  distilled  water,  this  cell  was  refilled 
with  a  fresh  normal  sugar  solution  and  placed  in  a  constant 
temperature  bath  at  35°.  The  fact  that  in  seven  days  it 
delivered  256.7  cc.  as  compared  with  226.2  cc.  for  the 
corresponding  period  of  the  first  test  shows  that  its  activity 
had  not  been  seriously  impaired.  It  also  confirms  observa- 
tions made  earlier  in  the  history  of  this  cell,  viz.,  that  the 
activity  is  notably  greater  at  higher  temperatures.  This 
work  was  done  in  early  January,  and  although  the  labora- 
tory temperature  approached  20°  in  the  afternoon,  a  some- 
what lower  temperature  was  the  rule  during  the  night  and 
the  morning  hours,  and  the  rate  of  delivery  as  indicated  by 
the  readings,  and  yet  more  strikingly  by  the  time  elapsing 
between  the  falling  of  successive  drops  from  the  delivery 


—  21  — 

tube,  followed  these  temperature  variations  quite  closely. 
The  following  figures  copied  from  the  notes  taken  at  the 
time  will  perhaps  be  of  interest.  They  cover  the  fourth, 
fifth,  and  sixth  days  of  this  test. 

Time.  Amt.  Delivered.  Temperature.  No.  of  Seconds. 

850A.M.  148.4   cc.  228 

10.30      "  150.5 

12.30  P.  M.  153.6 

4.45       "  159.7 

10.00  A.  M.  176.8 

2.00  P.  M.  181.5 

4.00       "  184.0 

10.45  A.  M.  201.0 

12.15  P.  M.  202.0 

2.45       "  204.8 


14°. 0     .  209 

17°.3  197 

19°.7  185.5 

13°.0  271.5 

16°.3  257 

19°.7  220 

15°.0  284 

16°.o  275 

18°.2  265 


Similar  readings  were  taken  covering  a  period  of  ten 
days,  indicating  the  same  dependence  of  rate  upon  tem- 
perature. Observations  taken  at  the  same  time  on  the 
action  of  other  cells  show  a  repetition  of  these  variations. 

This  membrane  was  not,  however,  possessed  of  indefinite 
endurance;  and,  after  ten  days  of  the  second  trial,  gave 
evident  signs  of  weakening.  It  was  accordingly  reinforced. 
A  battery  current  of  61.5  volts  was  employed  during  a 
period  of  two  hours,  at  the  end  of  which  time  the  steadily 
growing  resistance  registered  2010  ohms.  A  second  rein- 
forcement two  weeks  later  developed  a  maximum  resist- 
ance of  2040  ohms,  the  current  employed  being  taken 
from  the  dynamo  circuit  at  103  volts.  In  both  instances 
the  cell  fully  equaled  its  former  record  for  activity. 

In  a  second  cell  of  the  same  type  this  membrane  de- 
veloped a  resistance  of  1150  ohms  in  one  hour  and  thirty 
minutes,  the  battery  current  being  taken  at  38  volts.  When 
set  up  in  a  constant  temperature  bath  at  35°  the  delivery 
was  very  rapid,  9.9  cc.  in  one  hour,  109.6  cc.  in  one  day, 
and  275.4  cc.  in  seven  days.  One  hour's  reinforcement 
with  the  battery  current  at  62  volts  developed  a  resistance 
of  2033  ohms.  The  figures  for  delivery  under  the  same 


conditions  as  before  were:  10.0  cc.  in  the  first  hour,  111.2 
cc.  in  twenty -four  hours,  and  296.7  cc.  in  one  week.  When 
set  up  with  a  1.3  normal  sugar  solution,  the  delivery  was, 
in  two  hours,  18.9  cc.,  in  12  hours,  87.7  cc.,  and  in  seven 
days,  387.8  cc. 

The  interest  attaching  to  the  third  cell  in  which  this 
membrane  was  deposited  centres  mainly  in  the  fact  that 
this  cell,  despite  persistent  efforts,  had  previously  failed 
to  show  any  osmotic  activity  when  tried  with  the  ferric 
hydroxide  membrane.  In  this  test,  after  having  entirely 
removed  the  previous  deposits,  the  cobalt  ferrocyanide  mem- 
brane proved  as  satisfactory  as  in  the  two  previous  in- 
stances. A  resistance  of  1 100  ohms  was  reached  in  one  hour 
and  thirty  minutes,  and  the  membrane  showed  satisfactory 
activity  when  tried  with  solutions  of  both  sugar  and  alcohol, 
the  velocity  of  delivery  with  a  90. 5  per  cent  solution  of  alco- 
hol amounting  to  29  drops  per  minute,- 15.5  cc.  in  one  hour, 
130  cc.  in  twenty-four  hours.  This  proves  that  the  fail- 
ures of  three  cells  of  this  type  to  produce  a  satisfactory 
ferric  hydroxide  membrane,  cannot  be  explained  by  a 
chance  selection  of  three  cups  so  defective  that  they  could 
not  manifest  activity  with  any  membrane.  It  shows,  too, 
that  the  firmness  and  consistency  of  some  membranes  may 
enable  them  to  exhibit  good  results  in  walls  which,  with 
other  precipitates,  fail  completely.  We  also  saw  earlier 
that  the  ferric  hydroxide  precipitate,  which  requires  a 
quite  perfect  wall,  showed  remarkable  activity  in  a  cup 
which,  with  ferric  phosphate,  had  given  no  hope  of  suc- 
cess. 

This  membrane  was  later  reinforced  and  set  up  with  a 
thrice  normal  sodium  chloride  solution,  but  the  delivery 
was  quite  low;  only  18  cc.  in  twenty-four  hours.  The 
membrane  seemed  at  the  end  of  this  time  to  be  nearly 
destroyed,  and  the  outside  water,  which  then  contained 
scarce  more  than  a  trace  of  sodium  chloride,  was  in  another 


—  23  — 

twenty -four  hours  intensely  salt.  When,  after  prolonged 
washing,  the  reinforcement  of  the  membrane  was  again 
attempted  the  resistance  was  no  higher  than  when  begin- 
ning a  new  cell  in  which  no  membrane  had  ever  been 
deposited. 

A  fourth  cell,  of  the  same  type  but  harder  burned, 
showed  higher  resistance — 1500  ohms  in  the  first  treat- 
ment and  3050  and  3060  ohms  respectively  in  the  first  and 
second  reinforcements.  The  rapidity  of  flow  from  this 
cell  was  about  20  %  lower  than  that  recorded  in  the  case  of 
the  other  cells  with  the  same  membrane,  but  the  amounts 
of  sugar  or  alcohol  passing  through  the  walls  of  the  cell 
were  likewise  much  smaller.  On  this  subject  more  will  be 
said  in  the  general  discussion  later  on. 

The  Prussian  Blue  Membrane. — This  was  the  last 
membrane  taken  up  for  study  in  the  present  investigation. 
It  was  known  to  be  an  active  membrane,  and  its  capabili- 
ties had  been  somewhat  investigated ;  not  thoroughly,  how- 
ever, and  especially  not  in  the  light  of  the  electrolytic 
methods  of  Morse  and  Horn.  This  substance  is  a  well- 
known,  beautiful  blue  precipitate.  Its  ready  cohesion  and 
rather  firm  texture  highly  recommend  it ;  though  it  is  of 
course  to  be  expected  that  it  will  be  liable,  in  much  the  same 
way  as  other  ferrocyanides,  to  the  destructive  chemical 
action  of  some  of  the  substances  whose  osmotic  properties 
it  may  be  desirable  to  determine.  Here  again  a  low  vol- 
tage was  found  necessary,  particularly  at  starting.  In  a 
cell  of  the  coarser  grade  the  attempts  to  secure  a  satisfac- 
tory membrane  show  a  record  which,  it  is  thougtit,  will 
prove  not  uninteresting.  The  voltage  was  varied  between 
4.5  and  10  volts  with  a  N/8  solution  of  ferric  sulphate 
surrounding  the  cell  and  a  N/10  solution  of  potassium 
ferrocyanide  within.  After  two  hours  and  fifteen  minutes, 
a  resistance  of  80  ohms  was  indicated,  which  remained 
constant  during  an  hour  of  further  electrolytic  action. 


—  24  — 

Examination  of  the  cell  showed  that  all  the  precipitate  had 
been  formed  within  the  walls.  The  cell  when  set  up  with 
a  normal  sugar  solution  showed  no  activity,  and  the  follow- 
ing day  the  attempt  to  secure  a  membrane  was  repeated, 
the  concentration  of  the  outer  ferric  salt  being  made 
twice  as  great  as  on  the  former  occasion.  After  a  current 
of  12  volts  had  been  passed  through  the  cell  during  one 
hour  and  forty-five  minutes,  the  resistance  measured  only 
36  ohms.  The  precipitate  had  still  made  no  appearance 
on  either  surface  of  the  wall,  and  the  feeble  activity  of 
the  membrane  ceased  after  having  delivered  2  cc.  in 
twenty -four  hours.  The  difference  in  concentration  of  the 
electrolytes  was  still  further  exaggerated  so  that  now 
the  outer  iron  solution  was  N/2  and  the  potassium  fer- 
rocyanide  solution  N/20.  A  resistance  of  250  ohms  was 
reached  by  four  and  one-half  hours'  electrolysis.  The 
membrane  now  appeared  on  the  inner  wall,  and  when  set 
up,  its  activity  was  very  satisfactory  indeed,  a  rate  of 
about  6  cc.  per  hour  being  delivered  at  the  beginning. 
This,  however,  after  twenty  hours  was  reduced  to  0.4  cc. 
per  hour,  the  delivery  being  of  a  marked  blue  tint.  On 
the  fourth  attempt  the  deposition  of  the  membrane  was 
continued  two  hours  and  forty -five  minutes.  The  resist- 
ance was  255  ohms,  and  the  membrane,  though  less  active, 
was  more  permanent  than  before,  delivering  111  cc.  in 
thirteen  and  a  half  days. 

A  fifth  time  the  membrane-forming  process  was  repeated, 
using  the  same  potential,  12  volts,  and  a  resistance  of  285 
ohms  was  measured  at  the  end  of  two  hours'  treatment. 
The  activity  and  permanence  of  the  membrane  were  much 
as  before,  but  no  blue  color  appeared.  The  sixth  and 
seventh  efforts  represented  about  one  hour  each  of  elec- 
trolysis, at  potentials  of  62  and  105  volts,  and  resistances 
of  1080  and  1097  ohms  respectively  were  recorded.  In 
both  cases  the  membrane  was  quite  satisfactory,  both  in 


-  25  — 

point  of  permanence  and  of  activity.  In  the  broken  cell, 
the  two  membranes  formed  within  the  wall  are  plainly  to 
be  seen  and  the  membrane  on  the  inner  edge  of  the  wall  is 
of  great  thickness. 

With  the  second  cell,  again  of  the  coarser  type,  satis- 
factory results  were  more  quickly  obtained.  A  large 
amount  of  ferric  hydroxide  precipitate  had  been  deposited 
within  the  walls  of  this  cell  in  previous  fruitless  efforts  to 
form  an  active  membrane  of  that  material,  and  without 
removing  this,  the  formation  of  the  new  membrane  was 
begun.  In  order  to  secure  the  thickness  which  seemed 
necessary  in  this  membrane,  a  strong  current  was  employed, 
as  much  as  0.7  amperes  being  passed  through  the  cell.  As 
in  the  preceding  case,  the  results  were  all  unsatisfactory 
until  the  relative  concentrations  of  the  electrolytes  were 
made  widely  different.  The  concentration  of  the  inner 
electrolyte  was  kept  N/10  throughout ;  the  outer  solution  of 
ferric  sulphate  being  changed  from  N/5  to  N/2.  Under  the 
changed  conditions  the  resistance  rose  satisfactorily,  reach- 
ing 371  ohms  in  the  first  treatment,  1200  ohms  in  the  sec- 
ond, and  2240  ohms  in  the  third,  when,  finally,  a  very 
satisfactory  membrane  was  obtained.  Subsequent  exami- 
nation disclosed  the  fact  that  this  membrane  was  ex- 
tremely thick,  local  maxima  reaching  dimensions  of  0.7  mm. 

With  the  smaller  cells  resistances  of  17,000  and  54,000 
ohms  were  obtained.  These  membranes  were  active  with 
solutions  of  sugar,  alcohol  and  sulphuric  acid ;  but  in  the 
case  of  the  latter  the.  leakage  was  large  and  the  delivery 
soon  ceased. 

GENERAL    CONCLUSIONS. 

The  final  test  of  all  these  membranes  must  be  made  in 
cells  whose  walls  are  capable  of  giving  sufficient  support 
to  the  membrane  throughout  its  entire  area.  Until  such  a 
cell  is  obtained,  we  must  conclude  that  the  manganese  fer- 
rocyanide  membrane  is  unpromising  both  in  point  of  per- 


—  26  — 

manence  and  activity ;  that  the  ferric  phosphate  membrane 
is  purely  problematical ;  that  the  ferric  f errocyanide  mem- 
brane is  satisfactorily  active,  and  is  the  most  permanent  of 
the  f  errocyanide  membranes  here  tried;  that  the  cobalt 
ferrocyanide  has  sufficient  permanence  for  purposes  of 
measurement,  and  possesses  moreover  very  great  activity, 
together  with  a  firmness  and  consistency  far  superior  to 
those  of  other  membranes  experimented  with ;  finally,  that 
the  ferric  hydroxide  membrane  offers  both  permanence  and 
great  activity,  but  is  incapable  of  manifesting  the  latter 
property  except  in  cells  whose  walls  are  of  a  quite  uniformly 
fine  texture. 

We  may  now  turn  our  attention  to  the  consideration 
of  such  questions  as  whether  these  membranes  are  com- 
pletely or  only  partially  semipermeable,  i.  e.9  do  they  alto- 
gether prevent  or  merely  retard  the  passage  of  dissolved 
substances ;  whether  the  flow  of  water  through  the  mem- 
brane is  proportional  to  the  difference  in  concentration  of 
the  solutions  separated  by  the  membrane;  whether  the 
diminution  of  concentration  within  the  cell  bears  any  fixed 
ratio  to  the  volume  which  it  has  delivered  and  whether  the 
deficiencies  observed  are  due  to  defective  cell  walls;  and,  if 
so,  can  they  be  remedied. 

In  answering  the  first  question  it  must  be  said  that  in 
every  one  of  the  many  cases  observed  there  has  been 
leakage  through  the  walls  of  the  cell.  Hence,  either  the 
membrane  or  the  wall  is  defective,  perhaps  both.  The 
evidence  all  points  towards  the  deficiency  of  the  wall. 
This  evidence  is,  first,  the  fact  that  in  all  cases  in  which  the 
deposition  of  the  membrane  was  long  continued  a  maximum 
resistance  was  reached,  which  ordinarily  became  constant 
for  all  further  treatment.  This  fact  seems  capable  of 
explanation  only  on  the  supposition  that  there  is  a  constant 
cross-section  of  unimpeded  transference  of  the  ions ;  or,  in 
other  words,  a  number  of  pores  too  large  to  be  bridged  over 
by  the  membrane. 


—  27  — 

Second.  The  much  larger  leakage  when  the  cell  is  deliver- 
ing under  somewhat  greater  pressures.  Three  examples 
taken  from  a  number  of  these  observed  differences  will 
suffice. 

Cell  XXII.  was  tried  with  normal  sugar  solutions,  while 
in  cells  XVII.  and  XV.  exactly  60.0  gms.  of  sugar  was 
employed  in  each  instance.  The  periods  of  time,  pressures 
in  millimeters  of  water,  and  leakages  are  given  below. 

CELL   XXII. 

Time.  Pressure.  Leakage. 

7  da  ,  50  min.  10mm.  5.074  gms. 

7  "      70     "  74    "  7.052     «• 

CELL  XVII. 

Time.  Pressure.  Leakage. 

10  da.,  21.5  hrs.  10  mm.  4.532  gms. 

10  "      20       "  174  "  6.587     " 

CELL   XV. 

Time.  Pressure.  Leakage. 

10  da  ,  22  hrs.  10  mm,  4.354  gms. 

10  "      19  "  222  "  7.077     " 

Now,  if  the  membrane  is  intact  this  leakage  is  simply 
the  measure  of  the  diffusion  of  the  dissolved  substance ; 
and  the  rate  of  diffusion  of  a  given  substance  through  a 
given  medium,  the  temperature  remaining  unchanged,  de- 
pends only  upon  its  concentration.  But  the  concentra- 
tion is  not  appreciably  altered  by  these  slight  differences 
of  pressure.  Consequently,  not  diffusion  merely,  but  a 
positive  outward  flow  of  the  solution  takes  place.  Hence 
there  are  interstices  which  make  this  movement  possible. 

Third.  The  same  membrane,  cobalt  ferrocyanide,  pre- 
pared with  equal  care,  gives  in  different  cells  widely  dif- 
ferent amounts  of  leakage.  The  thickness  of  the  semi- 
permeable  membrane  should,  if  the  area  is  the  same,  vary 
with  the  number  of  coulombs  represented  by  its  formation. 
Hence,  in  cell  XXI.,  in  the  formation  of  whose  membrane 
0.19  ampere  hours  were  used,  the  membrane  should  be 
not  appreciably  thicker  thstn  in  cell  XIX.,  whose  mem- 
brane represents  0.17  ampere  hours  of  current.  And  if 


—  28  — 

the  leakage  is  to  be  explained  by  diffusion  there  should  be 
no  wide  difference  here.  Yet,  the  leakage  through  cell 
XXI.,  viz.,  0.8904  grms.  sugar  in  7  days,  23  hours,  was 
much  less  than  through  the  other  wall.  The  number  for 
this  is  5.458  grms.  sugar  in  12  days,  2^  hours.  Again, 
for  cell  XXI.  set  up  with  a  solution  containing  74.33  grms. 
alcohol  we  have  the  small  leakage  of  0.2077  grms.  in  11 
days,  23  hours;  while  in  cell  XX.  with  the  same  mem- 
brane and  set  up  with  127.2  grms.  of  alcohol  we  have  a 
leakage  of  3.540  grms.  in  10  days,  18  hours.  The  same 
cell,  XXI. —  with  the  advantage,  however,  of  a  reinforce- 
ment of  the  membrane — shows  a  smaller  leakage  with  the 
much  smaller  and  more  active  alcohol  molecule  than  that 
recorded  for  the  sugar ;  while  if  diffusion  were  the  explana- 
tion, the  opposite  result  should  have  been  observed.  The 
figures  for  cell  XXI.,  both  in  point  of  delivery  and 
smallness  of  leakage,  show  that  this  membrane  is  semi- 
permeable  quite  as  well  for  the  smaller  alcohol  molecule  as 
for  the  larger  molecule  of  sugar.  The  evidence,  such  as  it 
is,  favors  the  complete  semipermeability  of  the  membrane; 
though  a  demonstration  is  hardly  possible  with  the  means 
at  hand. 

The  answer  to  the  question:  Is  the  flow  through  the 
membrane,  of  the  pure  solvent  from  without,  proportional 
to  the  concentration  of  the  solution  within?  seems  to  be 
the  same  both  in  theory  and  practice.  The  rate  of  flow  in- 
creases with  higher  concentrations,  but  not  in  a  direct 
ratio.  This  is  to  be  expected  if  we  consider  that  the 
porous  wall  offers  a  very  high  resistance  to  the  rapid  pas- 
sage of  a  liquid.  Hence,  the  greater  the  concentration 
and  the  more  rapid  the  flow,  the  more  effectively  will  this 
resistance  be  felt.  Again,  the  rapid  influx  of  water 
through  the  membrane  dilutes  more  appreciably  the  solu- 
tion immediately  adjacent  to  the  membrane,  thus  reducing 
the  effective  concentration  of  the  solution,  and,  in  con- 


sequence,  the  flow  through  the  membrane.  Further,  there 
is  a  dependence  upon  the  relative  densities  of  the  solu- 
tions employed.  In  the  sugar  solution  the  density 
exceeds  that  of  the  outer  water,  and  the  dilution  of 
the  portion  adjacent  to  the  membrane  makes  this  part 
lighter  than  the  remaining  liquid,  giving,  in  consequence, 
upward  currents  along  the  cell  wall  toward  the  point 
of  delivery.  The  solution  delivered  is,  therefore,  of 
lower  density  than  the  average  concentration  remaining 
within  the  cell.  While  if  alcohol  be  used,  the  opposite 
results  must  take  place,  the  more  concentrated  solution 
being  first  delivered,  and  the  greater  density  of  the  lower 
concentration  adjacent  to  the  membrane  causing  this  to 
flow  downward.  All  these  will  have  their  influence  in 
producing  a  rate  of  delivery  somewhat  different  from 
that  which  should  result  from  a  uniform  concentration  of 
the  solution  within  the  cell.  If  now  we  turn  from  theory 
to  the  figures  observed,  we  find  the  flow  from  cell  XVI. 
at  a  concentration  N/45  to  amount  to  7.7  cc.  per  day. 
The  proportional  flow  for  a  normal  solution  would  be 
more  than  346  cc.  per  day  under  the  same  conditions  of 
temperature.  These  temperature  conditions  were  iden- 
tical when  seven  days  later  with  a  fresh  solution  of  normal 
sugar  this  same  cell  exhibited  an  initial  flow  of  293  cc.  per 
day  —  a  difference  quite  appreciable,  yet,  perhaps,  not 
so  great  as  we  should  expect  with  these  large  figures.  The 
measurements  from  which  comparisons  are  drawn,  must, 
unless  corrections  can  be  applied,  be  taken  when  the  outer 
solvent  is  pure.  For  instance,  in  cell  XVI.  at  a  concen- 
tration N/45  the  flow  was  twice  as  great  as  when  this 
concentration  was  N/61,  because  of  the  presence  of  a 
very  small  amount  of  sugar  in  the  outer  water  at  the  latter 
concentration.  This  same  cell  at  a  concentration  0.03 
grms.  per  cc.,  when  the  amount  of  sugar  in  the  outer 
water  had  risen  to  0.020  grms.  per  cc.,  trebled  its  flow  on 


—  30  — 

replacing  the  latter  by  pure  distilled  water.  The  differ- 
ence of  the  concentrations  within  and  without  had  been 
0.01  grms.  per  cc.  in  the  former  instance,  and  was  three 
times  that  after  the  change.  This  showed  that  at  low 
concentrations  the  flow  is  very  nearly  in  direct  proportion 
to  the  difference  of  concentration  of  the  inner  and  outer 
liquids;  also  that  unless  the  outer  water  is  pure,  or  its 
concentration  known,  the  flow  from  a  given  cell  cannot  be 
taken  as  a  measure  of  the  activity  of  its  membrane  at  the 
actual  concentration  of  the  solution  within ;  the  more  so  if 
this  concentration  be  rather  low,  for  then  the  concentra- 
tion of  the  outer  liquid  may  easily  represent  a  large  per- 
centage of  the  concentration  within,  and  the  flow  through 
the  membrane  will  be  diminished  in  the  same  ratio.  As 
the  leakage  from  the  cells  was  in  most  cases  allowed  to 
accumulate  and  no  accurate  knowledge  of  its  concentration 
was  attempted  until  several  days  had  passed,  the  many 
readings  taken  represent  accurately  the  relative  activities 
of  the  various  membranes  at  the  beginning  only  of  the 
record  and  at  the  end  when  it  was  corrected  for  the  outer 
concentration  then  measured.  Still  all  the  earlier  readings 
taken  while  the  concentration  of  the  outer  liquid  was  an 
inappreciable  fraction  of  the  high  concentration  within, 
are  close  approximations  to  the  correct  values.  Hence, 
the  curves  of  several  of  the  typical  cells  are  here  given. 
The  total  delivery  is  plotted  against  time  and  the  slope  of 
the  curve  represents  the  rate  of  flow  of  the  cell  for  the 
corresponding  time. 

Curve  I.  illustrates  the  action  of  a  cobalt  ferrocyanide 
membrane  in  cell  XIX.  when  set  up  in  a  constant  tempera- 
ture bath  at  35°.  The  sugar  solution  was  1.3  normal. 

Curve  II.  is  for  the  same  cell  at  the  same  temperature, 
but  with  a  solution  exactly  normal. 

Curve  III.  represents  the  activity  of  the  same  membrane 
in  Cell  XVI.  when  set  up  with  a  normal  sugar  solution  at 
the  temperature  of  the  laboratory  —  about  18°. 


—  31 


Curves  IV.  and  V.  are  for  the  Prussian  Blue  and  Man- 
ganese ferrocyanide  membranes.  The  conditions  are  the 
same  as  those  last  given.  These  three  are  good  types  of 
the  records  made  by  the  respective  membranes. 


—  32  — 


CONCENTRATION    CHANGES. 

The  relation  between  the  total  volume  delivered  and  the 
diminishing  concentration  within  the  cell  is  also  interest- 
ing. If  we  suppose  diffusion  to  prevent  differences  of  con- 
centration in  the  different  portions  of  the  cell,  this  relation 
is  easily  worked  out  as  follows  :  Let  V  =  the  capacity  of 
the  cell  and  v  the  total  volume  delivered.  Then  v  is  the 
independent  variable  and  the  concentration,  K9  is  a  func- 
tion of  v  and  may  be  written  K  '=  <f>(v).  The  total  dis- 
solved substance  in  the  cell  at  any  time  is,  then,  V  4>(v) 
the  amount  at  starting  being  equal  to  V  <f>(o).  Now  when 
v  receives  the  increment  dv,  the  substance  lost  by  the  cell 
is  (j>(v)dv. 

This  quantity  can  also  be  expressed  by  representing  the 
difference  between  the  total  amounts  of  dissolved  material 
in  the  cell  before  and  after  the  delivery  of  the  volume  dv. 
Hence  we  have 


=~ 


Whence  loge  <f>(v)  =  —       +  (7. 
Now  making  v  =  0,  we  have  loge  </>(0)  =  Cy, 
and  subtracting,  loge  </>(»  —  loge  <K°)  ==—      = 


But  the  ratio  "T/TvT  expresses  in  terms  of  the  original 

concentration  of  the  cell,  the  concentration  remaining  after 
the  volume  v  has  been  delivered.     Hence  we  may  write 

Log.  *•  =  —  •.   or, 


—  33  — 

Hence  the  concentration  of  the  cell,  taking  the  original 
concentration  as  unity,  is  represented  by  the  above 
fraction  —  in  which  e  is  the  base  of  the  Naperian  system 
of  logarithms  —  raised  to  the  power  which  is  expressed  by 
the  ratio  between  the  amount  delivered  and  the  capacity  of 
the  cell.  When  the  value  of  this  exponent  becomes 
1,  2,  3,  4,  etc.,  the  values  of  .fiTare  0.36788,  0.13533, 
0.049788,  0.018315,  etc.  We  have  seen  that  in  practice 
we  may  expect  these  values  to  be  exceeded  when  the 
density  of  the  solution  is  greater  than  that  of  the  solvent, 
and,  when  the  density  of  the  solvent  is  the  greater,  to  be 
not  attained.  t 

Leakage  is  another  factor  which  must  be  taken  into  con- 
sideration. Its  effect  is,  in  all  cases,  to  diminish  the  con- 
centration; and  this  must  be  borne  in  mind  whenever 
comparisons  are  made  between  the  results  above  deduced 
and  the  figures  actually  observed  in  the  laboratory.  If  the 
test  of  the  cell  cover  a  long  period  of  time,  as  in  the 
55  days  trial  of  cell  XVI.  this  factor  of  leakage  is  very 
important.  The  rate  of  flow  becomes  quite  small,  and  the 
amount  of  solution  passing  through  the  walls  may  reach  a 
magnitude  almost  equal  to  the  delivery.  In  this  case  the 
final  concentration,  instead  of  being  greater  than  the  figures 
above  given,  will  be  much  less.  The  figures  observed  for 
this  cell,  No.  XVI.,  are:  computed  residual  concentration 
after  a  delivery  of  574  cc.,  0.01541  grms.  per  cc. ;  actual 
0.00554  grms.  per  cc.  If  now  we  take  cell  XXI.  in 
which  the  leakages  of  both  sugar  and  alcohol  were 
so  small  as  to  influence  the  results  inappreciably,  we 
find  the  concentrations  in  the  case  of  the  sugar  solution  to 
be:  deduced,  0.3889  normal;  actual,  0.4261  normal. 
Here  the  time  was  seven  days,  twenty -three  hours,  and  the 
leakage,  0.8904  grms.  And  for  alcohol  —  the  time  being 
eleven  days,  twenty-three  hours,  and  the  leakage,  0.2077 
grms.  —  deduced,  0.1308  normal;  actual,  0.09698  normal. 

3 


—  34  — 

These  examples  show  clearly  that  the  upward  currents  of 
the  sugar  solution  when  diluted  at  the  cell  wall  by  the  en- 
tering water,  and  the  downward  currents  of  the  diluted 
alcohol  solution,  take  place  as  was  previously  surmised, 
and  that  a  liquid  of  less  density  than  the  average  concen- 
tration of  the  inner  solution,  is  constantly  being  delivered. 
This  is  clear  also  from  the  recorded  measurements  of  the 
concentrations  of  small  amounts  just  delivered,  and  of  the 
residual  liquids  in  the  cells.  In  cell  XXII.,  set  up  with 
sugar,  the  41.3  cc.,  just  delivered,  had  a  concentration  of 
0.1911  grms.  per  cc.,  while  the  average  concentration  of 
the  inner  liquid  after  delivering  the  above  was  0.2062 
grms.  per  cc.  And  for  cell  XX.,  set  up  with  alcohol,  the 
density  of  the  solution  just  delivered  was  0.9824,  as  con- 
trasted with  a  density  0.9891  for  the  liquid  within. 

THE    CELL    WALL. 

We  may  add  to  the  evidence  on  this  question  thus  far 
considered,  the  fact  that  out  of  a  lot  of  100  cells  which 
gave  the  best  results  in  all  work  done  in  this  laboratory,  a 
very  small  number —  the  least  porous  among  them  —  gave 
really  good  results.  Work  done  by  several  investigators  in 
the  earlier  part  of  the  present  year  led  to  the  conclusion 
that  the  unsatisfactory  work  was  due  to  deficiencies  in 
the  wall.  It  was  hoped  that  some  method  might  be  dis- 
covered of  remedying  these  defects.  The  first  attempt 
was  made  with  kaolin  in  an  extremely  fine  state  of  division 
and  suspended  in  a  liquid  which  was  forced  through  the 
walls  both  by  electro lytical  methods  and  by  pressure. 
This  deposit  was  by  no  means  firm  enough  to  give  the  de- 
sired support.  After  a  repetition  of  this  process,  the 
deposits  of  kaolin  were  cemented,  as  far  as  this  could  be 
accomplished,  by  a  superimposed  electrolytic  deposit  of 
barium  sulphate.  This  attempt  to  remedy  the  cell  was 
likewise  without  satisfactory  result.  The  purpose  of 


—  35  — 

the  next  step  was,  by  means  of  a  strong  current, 
to  completely  fill  the  porous  wall  with  the  barium 
sulphate  precipitate.  But  no  high  electrical  resist- 
ance could  be  obtained,  and  hence  it  seemed  im- 
possible to  entirely  fill  the  cell  with  this  material.  Efforts 
to  form  a  membrane  after  these  treatments  were  not  more 
successful  than  those  which  had  preceded.  The  membrane 
seemed  to  form  in  portions  of  the  cell  not  occupied  by  the 
other  materials.  The  appearence  of  the  cell  was  most 
unsatisfactory ;  the  membrane  appeared  in  great  blotches 
on  the  inner  and  outer  edges  of  the  wall,  and,  on  breaking 
the  latter,  it  was  found  in  all  possible  positions  within  the 
wall.  Efforts  were  also  made  to  block  these  pores  with 
some  one  of  the  various  membranes  tried,  but  with  a  very 
small  measure  of  success.  The  first  reinforcement  of  the 
membrane  usually  closed  some  of  the  pores,  not  stopped 
by  the  first  application  of  the  membrane-forming  process, 
and  this  was  evidenced  by  the  higher  electrical  resistance 
of  the  membrane,  as  well  as  by  a  decrease  in  the  amount 
of  leakage  through  the  cell  wall.  But  later  reinforcements 
seldom  added  appreciably  to  either  of  these  results,  and  in 
no  case  did  repetitions  of  the  membrane-forming  process, 
even  when  long  continued,  give  any  approximation  to  the 
desired  condition  of  complete  semipernieability.  Cell 
XXII.  in  which  the  manganese  ferrocyanide  membrane  was 
deposited,  gives  a  typical  record  of  what  was  accomplished 
by  these  efforts.  After  each  determination  of  the  leakage, 
the  membrane  was  thoroughly  reinforced  before  the  next 
determination  was  begun.  The  cell  was  set  up  each  time 
with  a  normal  sugar  solution,  the  amount  of  sugar  con- 
tained being  about  68.5  gms. 

Pressure.  Time.  Leakage. 

1st  test.  10mm.  8  da.,  16  hrs.  19.42  gms. 

2nd    "  "  "  7  «'  20  u  5.01     " 

3rd     "  '«    "  8  "    5    "  6.38     " 

4th     "  •'    "  7  "    1   "  5.07     " 

5th     "  74   «  7  "    1   "  7.05     " 


—  36  — 

Thus  every  attempt  to  form  a  sound  membrane  in  an 
imperfect  porous  wall  resulted  in  failure. 

ADDENDUM. 

Those  whose  interest  has  carried  them  thus  far  will  be 
gratified  to  learn  that  at  this  stage  of  the  work  Morse  and 
Frazer  carried  out  in  this  laboratory  a  thorough  micro- 
scopic investigation  of  the  cells  tried  the  previous  year 
and  found  to  give  either  good,  bad  or  indifferent  results. 
In  all  these  cases  the  character  of  the  cell  wall  was  found 
closely  related  to  the  reliability  of  the  cell  for  the  meas- 
urement of  high  pressures.  In  the  good  cells  there  were 
no  large  pores  present,  and  the  greater  or  smaller  number 
of  these  in  the  other  cells  could  be  inferred  before  exami- 
nation on  a  mere  inspection  of  the  record  of  the  cell. 

After  an  exhaustive  study  of  the  probable  causes  of  the 
existence  of  these  pores,  and  their  possible  remedies,  the 
above  named  workers,  after  assuring  themselves  that  the 
American  potters  would  not  attempt  this  problem,  set 
themselves  the  task  of  producing  the  desired  perfect  wall. 
The  extreme  care  shown  in  all  details,  the  method  and 
thoroughness  of  the  work  thus  far  carried  out,  promise 
early  and  complete  success.  The  solving  of  this  problem 
will,  it  is  confidently  hoped,  mark  the  beginning  of  real 
quantitative  work  in  the  field  of  osmotic  phenomena.  And 
this,  in  turn,  will  lead  to  the  unraveling  of  some  very 
important  and  intricate  problems,  the  solution  of  which, 
depends  upon  this  accurate  knowledge  of  osmotic  activity. 


BIOGRAPHY. 

The  writer  was  born  in  Edina,  Missouri,  July  22,  1862. 
His  early  education  was  received  in  the  parochial  schools 
of  his  native  town,  and  from  private  tutors.  In  1884  he* 
entered  St.  Mary's  College,  Kansas,  and  in  1886-7  he 
taught  in  the  Grammar  Department  of  St.  Francis'  In- 
stitute, Osage  Mission,  Kansas.  With  the  exception  of 
two  years  spent  in  Theological  and  Biblical  studies  at 
Woodstock  College,  Maryland,  the  next  fourteen  years 
were  spent  by  him  in  following  the  Ascetical,  Classical, 
Scientific,  Metaphysical,  Ethical,  Exegetical  and  Theolog- 
ical courses  offered  in  the  Normal  and  Graduate  schools  of 
the  St.  Louis  University,  St.  Louis,  Missouri.  During 
four  years  of  this  period,  he  also  occupied  the  position  of 
Assistant  in  Mathematics.  Since  October  1,  1901,  he  has 
pursued  graduate  studies  in  Chemistry  at  the  Johns  Hopkins 
University.  His  subordinate  subjects  are  Physical  Chem- 
istry and  Geology. 

(37) 


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